Prior art electro-optical apparatus for the determination of the position of a rotational objects is shown in FIG. 1. A light-opaque mask 12 having light-transparent apertures 20 is rigidly attached to and moves with an object 22, whose angular position is to be determined. Light rays emitted from a light source 10, impinge on the mask 12 and a light flux passing through the apertures in the plate is detected by a light detecting device 14, which is aligned with the mask and the source to measure the light flux.
As different apertures 20, in the mask 12 pass in front of detector 14, an output signal is obtained from detector 14 which determines the position of object 22 with the aid of a position reporting device 18.
The problems associated with this type of apparatus and this method for position determination relate principally to limitations in position determination accuracy. Best positional accuracy obtained with this method and similar prior art methods is of the order of 1 micrometer.
The reasons for this accuracy limitation can be traced to the fact that light passing through an aperture and detected by a light sensor is registered either as a zero or as a one, i.e., there are no fractional measurements; thus, the positional accuracy is determined only by detection and identification of a particular slit through which light passes. Thus, in order to determine very small distance displacements the aperture width and the aperture intervals must be made impossibly small. Physical size, therefore limits the fine range measurements.
Other systems are capable of finding a position of an edge to greater accuracy than the size of the detector by utilizing digitized values at a number of positions as a single detector moves past the edge. The values are used to find the position of the edge. However, such methods are capable of less than an order of magnitude of improvement of resolution and furthermore, they do not measure the position at a point without making measurements at adjacent points. Thus the position of the edge is known only after the detector has moved away from the edge and is therefor not known in real-time, i.e., while the measurement is being made. Such determination of edges is used, for example, in the inspection of reticules for microcircuit production.
The apparatus of FIG. 1 is also sensitive to vibrations which can lead to large errors in position determination. Such prior art apparatus and similar apparatus (as described for example in Japanese Patent Publication No. He-1-22884) is, thus, not suitable for very precise, high resolution position determination.
For more precise positional accuracy determination, of better than 1 micrometer, most prior art devices rely on optical interferometry, which suffers from high sensitivity to vibration. Furthermore, such interferometers are high cost, very delicate apparatus. Examples of such apparatus are described in the General Catalog of Dr. Johannes Heidenhain, GmbH of Traunreut Germany dated June 1996, in particular on pages 6 and 7.